1. Field of the Invention
The present invention concerns fiber optic data transmission cables and in particular a fiber optic cable containing no reinforcing members.
2. Description of the Prior Art
Fiber optic cables are increasingly being used in data transmission, either as overhead cables or as buried cables. Regardless of the application, one essential component of a fiber optic cable is a central/or peripheral reinforcing member one function of which is to withstand tension loads applied to the cable during its installation. One such cable is described in U.S. Pat. No. 4,693,551.
Another essential function of the central and/or peripheral reinforcing member is to prevent curvature of the optical fibers due to temperature variations and likely to lead to loss of quality in data transmission. The optical fibers have a low coefficient of thermal expansion and their length therefore varies only slightly as the temperature falls (or rises), whereas the length of the plastics material surrounding them tends to vary by a large amount in this situation, because of its higher coefficient of thermal expansion. It is for this reason that in fiber optic cables the reinforcing member such as that shown in the patent mentioned hereinabove, which has a coefficient of thermal expansion approximately equal to that of the optical fibers, serves to limit variations in the length of the cable in the event of variations in temperature. A reinforcing member of this kind is generally made of metal or of a plastics material reinforced with glass fibers in the case of dielectric cables.
Moreover, when a reinforcing member is present, and still with the aim of preventing inappropriate curvature of the optical fibers, the latter are generally loose in their sheath, as shown in the U.S. patent previously mentioned. This loose structure allows the sheath to expand or contract freely without stressing the optical fibers and thus causing them to become curved.
In addition to a central reinforcing member, the use of a sheath of aramide fibers (sold under the trademark "Kevlar") at the periphery of the cable is known in itself. In the extreme situation, given that the optical fibers are mounted loose in the cable as previously explained, and provided that a large quantity of aramide fibers is used, it would be possible to dispense with any rigid central reinforcing member.
Unfortunately, the use of reinforcing members is costly, particularly in the case of dielectric cables when the reinforcing member is of a plastics material reinforced with glass fibers and/or in the case of aramide fiber sheaths, and this considerably increases the manufacturing cost of the fiber optic cable.
For this reasons, the aim of the invention is to provide a fiber optic cable that does not contain any central and/or peripheral reinforcing member made of metal or of a plastics material reinforced with glass fibers.